Molecular Breeding 4: 381–393, 1998. © 1998 Kluwer Academic Publishers. Printed in the Netherlands. 381 Identification of a QTL decreasing yield in barley linked to Mlo powdery mildew resistance W.T.B. Thomas * , E. Baird, J.D. Fuller, P. Lawrence, G.R. Young, J. Russell, L. Ramsay, R. Waugh and W. Powell Scottish Crop Research Institute, Invergowrie, Dundee DD2 5DA, UK ( * author for correspondence; e-mail: wthoma@scri.sari.ac.uk). Received 15 December 1997; accepted in revised form 7 May 1998 Key words: barley, QTLs, SSRs, yield, Mlo mildew resistance, introgression Abstract A molecular marker map, including Mlo mildew resistance, of the spring barley cross Derkado (Mlo-resistant) × B83-12/21/5 (Mlo-susceptible) was scanned for yield QTLs to determine whether the association of Mlo resistance with reduced yield was due to linkage or pleiotropy. Over the mapped portion of the genome of the cross, the QTL with the greatest effect upon yield was located within a 22 cM region between mlo and the simple sequence repeat HVM67 on chromosome 4(4H). The association of Mlo resistance with lower yield was therefore due to a repulsion linkage. Analysis of yield component characters revealed QTL alleles for reduced grain number and earlier heading date in the same region, also associated with Mlo resistance. Genotyping of a range of cultivars and sources of Mlo resistance with the HVM67 simple sequence repeat showed that the Derkado HVM67 allele was rare as it was found only in one other cultivar and four land-races or sources of disease resistance. Grannenlose Zweizeilige, the source, and Salome, the carrier of Mlo resistance in Derkado, have the same HVM67 genotype, although Salome was a mixture of two genotypes. The entire mlo-HVM67 chromosomal segment from Grannenlose Zweizeilige is therefore thought to have been transmitted to Derkado, possibly through joint selection for Mlo resistance and early heading. L92, synonym EP79, was another source of Mlo resistance with the same HVM67 allele as Derkado but recombination must have occurred during the breeding of Atem as it possesses a different HVM67 allele which is present in all the other Mlo sources and cultivars surveyed. Abbreviations: GN, grains per main stem ear; HD, heading date; MSTGW, thousand grain weight derived from GN and MSY; MSY, yield of grain on the main stem; PY, yield of grain from the whole plot; sCIM, simplified compound interval mapping; SIM, simple interval mapping; SPY, single plant yield; S-SAP, sequence-specific amplification polymorphism; TGW, thousand grain weight derived from bulk of plot seed; TN, number of fertile stems per plant. Introduction Breeding programmes usually attempt to combine high yield potential with disease resistance. In the UK barley crop, powdery mildew, caused by the fungus Erysiphe (= Blumeria) graminis f.sp. hordei, is the most important foliar disease. To provide genetic re- sistance to powdery mildew, breeders have introduced a number of resistance genes, mainly alleles at the Mla locus on barley chromosome 5 (1H), from unadapted germplasm. The use of unadapted germplasm as a source of disease resistance can also introduce detri- mental characteristics. For example, the introduction of the La mildew resistance from Hordeum laeviga- tum adversely affected malting quality in all the early cultivars that carried the resistance [44]. Many of the barley powdery mildew resistance genes have had a short effective commercial life as new races of the pathogen have evolved which can overcome the new